CORUNDUM-SYENITE. 201 



in these groups adjacent but isolated sections may show simul- 

 taneous extinction. The crystals are cracked in characteristic 

 fashion, with the development of yellow serpentine. The most 

 interesting feature in connection with the mineral is the occasional 

 occurrence of a narrow zone of rhombic pyroxene following the 

 outlines of the olivine and enclosing it partially in a shell, a feature 

 which has also been recorded for the olivine-bearing augite-syenite 

 of South Norway. 



The remaining constituents call for no special remark. The iron* 

 ores are the only ones occurring in considerable abundance. 



3.— Felspar Rock with Corundum 

 ( Corundum-syenite. ) 

 tehpar-rock is about the most expressive name to apply to 

 the rock which is found in such large quantities around each of the 

 small hillocks of elaeolite-syenite, as well as at the foot of, and some- 

 times at considerable heights on, Sivamalai itself ; but neither its 

 precise affinities to the elaeolite-syenite and augite-syenite, nor all 

 its curious variations have been worked out. It is, however, almost 

 certainly a genetic relative of both, due to differentiation of the 

 magma. On account of its general occurrence on the low ground it 

 is covered with detrital material which is often cultivated. This 

 rock occurs in two degrees of texture — (1) a medium-grained and 

 granulitic form, traversed by (2) its own coarse-grained pegmatite. 

 They agree approximately in composition, being composed largely 

 of felspar which in the granulitic form is microperthite and in {he 

 coarser type is an irregular intergrowth of albite and orthoclase. The 

 finer-grained form often contains a red garnet, which contains 

 much magnesia besides iron and alumina, 1 magnetite and other 



1 I have been unable to obtain sufficient material for a quantitative analysis 

 of this garnet, but the qualitative tests show that it is a mixture of pyrope and 

 almandine similar to the variety rhodolite which has been found by Judd and 

 Hidden to be intimately associated with ruby-corundum in North Carolina {Mitt. 

 Mag., XII, 1899, 145). 



( 33 ) 



